[tz] Dealing with Pre-1970 Data

Lester Caine lester at lsces.co.uk
Sun Sep 1 14:17:46 UTC 2013

David Patte ₯ wrote:
> On 2013-09-01 6:35, Zefram wrote:
>>> However the one I was really thinking about was the 'equation of
>>> >time' calculation in a format we can use as a 'standard'.
> Equation of time calculations are non-trivial requiring the knowledge of the
> variations of the orbit of the earth (to calculate sun position), as well as
> variations in earth revolution speed (to calculate DeltaT). For accuracy,
> astronomical software often uses large tables and series of thousands of
> polynomials to estimate local apparent time in the past. This is clearly outside
> the scope of a tz database.
> There are simplifications to these formulas, but they are only accurate for
> short timeframes.

David I appreciate your insight here. My own naive view on this was that in 
essence we only needed an equation for over the year, with a fiddle factor which 
my current crude scribblings suggests would be OK in 10 year steps and probably 
longer than that? So what is your definition of 'short'? I'm only looking back 
to 1600's myself and it's unlikely I'll get that far in my life time although 
discussions on the time machine are progressing well on one of the other lists I 
frequent :)

Lester Caine - G8HFL
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